1. Field of the Invention
This invention relates to a battery system, specifically to a battery system provided with a generator using a renewable energy as a solar battery, a DC/AC converter, a battery module and the like.
2. Description of the Related Art
This kind of the battery system is to provide a load with electric power by superimposing electric power generated by a solar battery or electric power discharged from a battery module on an indoor alternating current branch, which is supplied electric power from a commercial power grid. Power consumption from the grid can be reduced by an amount of the generated or discharged electric power, and maximum electric power to be supplied from the grid can be reduced as well.
In this case, since the electric power generated by the solar battery and the electric power discharged from the battery module are DC power, they are superimposed on the indoor alternating current branch after being converted into AC power of the same frequency as a frequency of the grid of alternating current through a DC/AC converter (inverter) provided between the solar battery or the battery module and the grid (Refer to Japanese Patent Application Publication No. H06-266458.).
Also, since the electric power generated by the solar battery varies depending on a changing amount of sun light, it is known that the DC/AC converter for solar battery (power conditioner) is provided with a DC/DC circuit that boosts the DC voltage before the DC/AC conversion so that the voltage and the current after the boosting are adjusted so as to maximize the electric power generated by the solar battery (Refer to Japanese Patent Application Publication No. 2002-354677.).
In addition, Japanese Patent Application Publication No. 2002-354677 discloses a structure in which a DC/DC converter (charger) is connected between the battery module and a connecting node between the DC/AC converter (inverter) and the DC/DC circuit (booster circuit) in the power conditioner. The DC/DC converter (charger) is configured to control so that the output voltage of the solar battery outputted through the power conditioner is stepped up or stepped down and outputted to the battery module, or so that the DC power from the battery module is kept stable.
However, an battery system disclosed in Japanese Patent Application Publication 2002-354677 is structured to charge the battery module with the electric power from the solar battery always through the DC/DC converter (charger).
That results in a reduction in charging efficiency due to a conversion loss of the DC/DC converter (charger) always when the solar battery charges the battery module, regardless of states of the solar battery and the battery module.
This invention is directed to solve the problem addressed above, and offers an battery system intended to reduce the conversion loss by the charger to reduce the power consumption.